The present invention relates to a rotation angle detection circuit for a step motor which is able to correctly detect the rotation angle of a rotary shaft of a step motor.
In general, a step motor is driven in an open loop type without a feedback because its control characteristic is extraordinarily good.
FIG. 1 is a control block diagram of a conventional step motor. As shown in the drawing, when a control signal is input to a drive unit 1, the drive unit 1 applies a predetermined pulse signal to a number of input terminals of a step motor 2. As a rotary shaft of the step motor 2 is rotated a required angle. However, in such a conventional step motor, there is no device for determining whether the rotary shaft of the step motor 2 is rotated as much as required.
This is explained in more detail with reference to the equivalent circuit of FIG. 2 in which a step motor provided with three input terminals A, B, C is illustrated as an example. As shown in the drawing, coils L1, L2; L3, L4; L5, L6 are connected in series to the input terminals A, B, C, with a predetermined angle kept.
In such a step motor, when power is applied selectively to the input terminals A, B, C by the selection of switches S1, S2, S3, a magnetic field is generated at the coils L1-L2; L3, L4; L5, L6 in response to the input terminals S1, S2, S3 to which the power is input. Thus a permanent magnet of a rotator is then rotated a short distance so as to be balanced with the generated magnetic field.
FIG. 3 is a diagram showing the rotation angle of the above-mentioned step motor. For example, under the state of "001" power is applied only to an input terminal A; when power is also applied to an input terminal B, the state changes to 011 and, a rotary shaft is rotated at an angle of +15.degree., and in this state 011 when power is applied only to the input terminal B, the state changes to "010", the rotary shaft is rotated at an angle of +15.degree..
As above, the rotator of the step motor is rotated at an angle of .+-.15.degree. or .+-.30.degree. in accordance with a state and power that is applied to the input terminals A, B, C of the step motor or is to be applied thereto.
However, in such a conventional step motor, since the rotation of the step motor is controlled in an open loop manner, it is difficult to determine how much the rotary shaft of the step motor is rotated, and thus there is a disadvantage in that it is impossible to use the step motor in a system which needs a position signal of the rotary shaft of the step motor.